Re: [LSF/MM/BPF TOPIC] Locally attached memory tiering

["Huang, Ying" <ying.huang@intel.com>]

Hi, David,

Thanks!  This is a great summary!

David Rientjes <rientjes@google.com> writes:

> Hi all,
>
> I think it would be very worthwhile to have a block set aside for 
> discussion on locally attached memory tiering extensions at LSF/MM/BPF 
> 2024.
>
> Primarily interested in discussing Linux enlightenment for CXL 1.1 and 
> later type-3 memory expansion devices (CXL.mem).  I think we could touch 
> on CXL 2.0 and later memory pooling architectures if we have time and 
> there is interest, but the primary focus here would be local attached.
>
> Based on the premise for a Memory Tiering Working Group[1], there is 
> widespread interest in the foundational topics for generally useful Linux 
> enlightenment:
>
>  - Decoupling CPU balancing from memory balancing (or obsoleting CPU
>    balancing entirely)
>
>    + John Hubbard notes this would be useful for GPUs:
>
>       a) GPUs have their own processors that are invisible to the kernel's
>          NUMA "which tasks are active on which NUMA nodes" calculations,
>          and
>
>       b) Similar to where CXL is generally going, we have already built
>          fully memory-coherent hardware, which include memory-only NUMA
>          nodes.
>
>  - In-kernel hot memory abstraction, informed by hardware hinting drivers
>    (incl some architectures like Power10), usable as a NUMA Balancing
>    backend for promotion and other areas of the kernel like transparent
>    hugepage utilization
>
>  - NUMA and memory tiering enlightenment for accelerators, such as for
>    optimal use of GPU memory, extremely important for a cloud provider
>    (hint hint :)
>
>  - Asynchronous memory promotion independent of task_numa_fault() while
>    considering the cost of page migration (due to identifying cold memory)
>
>  - What the role of userspace plays in this decision-making and how we can
>    extend the default policy and mechanisms in the kernel to allow for it
>    if necessary
>
> Additional topics that you find interesting are also very helpful!

In addition to the hot memory identification and promotion, I think that
we should consider the cold memory identification and demotion too as a
full solution.  The existing method based on the page table accessed bit
may be good enough, but we still need to consider the full solution in
the context of the general NUMA balancing.

> I'm biased toward a generally useful solution that would leverage the 
> kernel as the ultimate source of truth for page hotness that can be 
> extended for multiple use caes, one of which is memory tiering support.  
> But certainly if there are other approaches, we can discuss that as well.
>
> A few main goals from this discussion:
>
>  - Ensure that proposals address, or can be extended to address, the 
>    emerging needs of the various use cases that users may have
>
>  - Surface any constraints that stakeholders may find to be prohibitive
>    for support in the core MM subsystem
>
>  - Alignment and division of work for developers who are actively looking
>    to contribute to this area
>
> As I'm just one of many stakeholders for this discussion, I'd nominate 
> Michal Hocko to moderate it if he's willing to do so.  If he's so willing, 
> we'd be in good hands :)
>
>  [1] https://lore.kernel.org/linux-mm/45d850ec-623b-7c07-c266-e948cdbf1f62@linux.com/T/

--
Best Regards,
Huang, Ying